Linear motion device

ABSTRACT

A linear motion device has: a linear motion body externally fitted onto a shaft and making a relative linear motion along the shaft; a plurality of balls retained in a rolling element groove formed in an inner surface of the linear motion body, and rolling between the rolling element groove and the shaft; separators interposed among the balls; and a circulating path formed in the linear motion body and for circulating the balls from one end side of the rolling element groove to the other end side thereof; wherein the linear motion device is charged with grease containing base oil which is 220-395 in worked penetration defined by JIS K2220 and 50-500 mm 2 /s in kinematic viscosity at 40° C.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to linear motion devices incorporated intoindustrial machines, such as ball screw devices, linear guide devices,linear ball bearing devices and ball spline devices. Of these linearmotion devices, the present invention particularly relates to atechnique for improving the durability of linear motion devices such asinjection molder driving ball screws or machine tool ball screws usedwith grease lubrication, and vibration absorbers, when the linear motiondevices are used in high contact pressure conditions.

2. Description of the Related Art

Linear motion devices using balls as rolling elements are conventionalused. For example, ball screw devices are used for converting a rotarymotion into a linear motion in machine tools such as machining centersor industrial robots, and linear guide devices are used for smoothlinear movement of work tables or the like.

FIG. 1 is a top view showing an example of a ball screw device which isone of linear motion devices. FIG. 2 is a sectional view taken on lineA-A in FIG. 1. As illustrated, the ball screw device has a screw shaft2, a cylindrical nut 4 and a plurality of balls 5 as its chiefconstituent members. A male thread groove 1 is formed spirally in theouter circumference of the screw shaft 2, while a female thread groove 3is formed spirally in the inner circumference of the nut 4 so as to beopposed to the male thread groove 1. The balls 5 are interposed betweenthe male thread groove 1 and the female thread groove 3. A flange 6 tobe fixed to a not-shown table or the like is formed in one end of thenut 4 while a flat surface (notched surface) 7 is cut in a part (upperin FIG. 2) of the outer circumferential surface of the nut 4. A pair offront and rear steel tubes 8 are fixed to the nut 4 so as to serve ascirculating paths for the balls 5. Thus, there is formed a structure inwhich the balls 5 making 3.5 rotations between the thread grooves 1 and3 circulate through the tubes 8. Incidentally, in FIGS. 1 and 2, thereference numeral 9 represents a tube clamp for fixing the tubes 8 ontothe flat surface 7 of the nut 4, and 10 represents a dust-proof plasticseal attached to each end of the nut 4.

Generally, the ball screw device in FIG. 1 is also referred to as a tubemethod because it uses the tubes 8 as circulating paths. As for thecirculating system, there are some systems other than the tube method,such as a deflector method (FIG. 3) using deflectors 11 as circulatingpaths, and an end cap method (FIG. 4) using end caps 12 as circulatingpaths. Most of ball screw devices for use in high-load applicationsadopt the tube method.

FIG. 5 shows a linear guide device which is one of linear motiondevices. As illustrated, the linear guide device has a bearing 14, arail 16 and a plurality of balls 17 as its chief constituent members.The bearing 14 has a bearing raceway groove 13 formed in its innersurface. A rail raceway groove 15 opposed to the bearing raceway grooveis formed in the outer surface of the rail 16. The balls 17 areinterposed between the bearing raceway groove and the rail racewaygroove. An end cap 18 is attached to each end of the bearing raceway soas to form a circulating path in cooperation with a circulatingpassageway 19 formed in the bearing. Thus, there is formed a structurein which the plurality of rolling elements can circulate.

In addition, grease is often used for lubrication in such linear motiondevices. For example, it is typical to adopt a method for frequentlyintermittently feeding mineral-lithium-based extreme pressure greaseusing lithium or lithium complex soap as thickener and mixed with anextreme pressure agent.

Such a linear motion device has an advantage in that the drivingefficiency is extremely high because the linear motion device supports aload through balls and friction loss during operation is thereforenearly negligible, or much smaller than in a corresponding sliding screwor a corresponding sliding support member. In addition, the linearmotion device has advantages in that the friction among the constituentmembers is extremely slight and the mechanical efficiency is highbecause the relative rotation between the screw shaft 2 and the nut 4 isnot accompanied by sliding.

However, the field of applications of linear motion devices high inmechanical efficiency has been expanded with the recent tendency ofpower saving in machinery and equipment. Thus, the working conditions oflinear motion devices are becoming more and more harsh. For example, asfor the case of ball screw devices, there is increasing use of a ballscrew device even in a driving unit of an apparatus requiring very highdriving power, such as an injection molder or a pressing machine whichhas been driven by a hydraulic system in the related art. In such a ballscrew device used in high load conditions, indentation or peeling is aptto occur in the ball surface so that the ball screw device has to beexchanged more frequently. In addition, grease should be fed at ashorter interval so that the grease consumption increases.

In addition, there is increase use of linear guide devices in quakeabsorbing mechanisms for housing. A very high load is applied to linearguide devices used for supporting buildings in comparison with linearguide devices used in machine tools. In addition, once installed in thebuildings, most of the linear guide devices get along withoutmaintenance over a long term. Thus, such linear guide devices arerequired to have high durability.

In order to improve the durability of a linear motion device, varioustechniques have been proposed in the related art. For example, there isknown a ball screw device in which spacers each having a specific shapeare interposed among balls (e.g. see JP-A-2001-21018); a ball screwdevice in which spacers having central positions displaced from oneanother and each having an arc surface like a Gothic arch are interposedamong balls (e.g. see JP-A-2000-120825); a technique using a low-dustlubricant using base oil with defined surface tension at 25° C. (e.g.see JP-A-2001-59094); or a technique covering a contact surface with ahigh hard film (e.g. see JP-A-2000-205280).

SUMMARY OF THE INVENTION

The present invention was developed in consideration of suchcircumstances. It is an object of the present invention to provide alinear motion device superior in durability in which device occurrenceof indentation or peeling is suppressed in the ball surface and theraceway surface even if the linear motion device is used in high contactpressure conditions.

In order to attain the foregoing object, the present invention providesthe following linear motion devices.

A linear motion device including: a linear motion body externally fittedonto a shaft and making a relative linear motion along the shaft; aplurality of balls retained in a rolling element groove formed in aninner surface of the linear motion body, and rolling between the rollingelement groove and the shaft; separators interposed among the balls; anda circulating path formed in the linear motion body and for circulatingthe balls from one end side of the rolling element groove to the otherend side thereof; wherein the linear motion device is charged withgrease containing base oil which is 220-395 in worked penetrationdefined by JIS K2220 and 50-500 mm²/s in kinematic viscosity at 40° C.

In the above construction, it is preferable that surfaces of each of theseparators in contact with the balls are concave surfaces each formedwith a curvature radius larger than a radius of each of the balls, and athrough hole formed to penetrate each of the separators from one of theconcave surfaces to the other of the concave surfaces is formed in acentral portion of the separator, while lips made of separate membersare provided additionally to the separator at a plurality of locationsin a circumferential edge of each of the concave surfaces so as toextend outward.

A linear motion device including: a linear motion body externally fittedonto a shaft and making a relative linear motion along the shaft; aplurality of balls retained in a rolling element groove formed in aninner surface of the linear motion body, and rolling between the rollingelement groove and the shaft; a circulating path formed in the linearmotion body and for circulating the balls from one end side of therolling element groove to the other end side thereof; and separatorsinterposed among the balls; wherein each of the separators has contactsurfaces each contacting with the ball and having a concave surface likean offset Gothic arch, a ratio f_(p)=r_(p)/D_(w) of a curvature radiusr_(p) of the concave surface to a ball diameter D_(w) is in a range offrom 0.53 to 0.67, a ratio Γ=A_(p)/D_(w) of an offset A_(p) of theGothic arch to the ball diameter D_(w) is in a range of from3.486×10⁻¹f_(p)−1.743×10⁻¹ to 3.390f_(p)−1.700, and a ratio θ=d/D_(w) ofan outer diameter d of the separator to the ball diameter D_(w) is in arange of from 0.65 to 0.85, or each of the separator has concavesurfaces each contacting with the ball, the concave surface being aconical surface shape with an apex angle α not smaller than 64° and notlarger than 140°, and a ratio θ=d/D_(w) of an outer diameter d of theseparator to the ball diameter D_(w) is in a range of from 0.65 to 0.85;wherein the linear motion device is charged with grease containing baseoil and thickener, the base oil being 50-500 mm²/s in kinematicviscosity at 40° C., the thickener being contained at 3-40% by mass withrespect to whole mass of the grease so that worked penetration definedby JIS K2220 is 220-395.

In the above construction, it is preferable that the grease contains atleast one kind of extreme pressure agent selected from an organic nickelcompound, an organic molybdenum compound and an organic telluriumcompound at 0.1-15% by mass with respect to whole mass of the grease.

In the above construction, it is preferable that a ratio f=r/D_(w) of arolling element groove radius r to the ball diameter D_(w) is in a rangeof from 0.505 to 0.550.

In the above construction, it is preferable that the linear motiondevice is a ball screw device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view showing an example of a tube-method ball screwdevice which is a kind of linear motion device according to the presentinvention;

FIG. 2 is a sectional view taken on line A-A in FIG. 1;

FIG. 3 is a top view showing an example of a deflector method ball screwdevice which is a kind of linear motion device according to the presentinvention;

FIG. 4 is a top view showing an example of an end-cap-method ball screwdevice which is a kind of linear motion device according to the presentinvention;

FIG. 5 is a partially cutaway perspective view showing an example of alinear guide device which is a kind of linear motion device according tothe present invention;

FIG. 6 is a partially enlarged view showing the tube-method ball screwdevice shown in FIG. 1, along its screw shaft;

FIG. 7 is a sectional view showing a preferred embodiment of aseparator;

FIG. 8 is a sectional view showing another preferred embodiment of theseparator;

FIG. 9 is a sectional view showing a further preferred embodiment of theseparator;

FIG. 10 is a sectional view showing the further preferred embodiment ofthe separator;

FIG. 11 is a sectional view showing a still further preferred embodimentof the separator;

FIGS. 12A and 12B are sectional views showing another preferredembodiment of the separator;

FIGS. 13A to 13D are sectional views showing a further preferredembodiment of the separator;

FIGS. 14A to 14D are sectional views showing a still further preferredembodiment of the separator;

FIG. 15 is a graph showing the conditions the values f_(p) and F of theseparator should satisfy; and

FIG. 16 is a perspective view showing an example of a linear ballbearing device which is a kind of linear motion device according to thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Detailed description will be made below on linear motion devicesaccording to the present invention.

In the present invention, the kind of linear motion device and itsspecific configuration or structure are not limited particularly. Thepresent invention is applicable to ball screw devices, linear guidedevices, linear ball bearing devices, ball spline devices, and the like.

FIGS. 1 to 4 shows the configurations of ball screw devices by way ofexample. Incidentally, according to the present invention, separators 21are interposed among balls 5 as shown in FIG. 6 which is a partiallyenlarged sectional view along two thread grooves 1 and 3 in FIG. 1. Theseparators 21 move between the two thread grooves 1 and 3 or in theinside of tubes 8 along with rolling of the balls 5.

In such a ball screw device, when a screw shaft 2 is driven to rotate bya not-shown motor or the like, a nut 4 screwed down to the screw shaft 2through the balls 5 screws in the front/rear direction (left/rightdirection in FIG. 1). On that occasion, the male thread groove 1 on thescrew shaft 2 side and the female thread groove 3 on the nut 4 siderotate relatively in directions reverse to each other. Thus, the balls 5roll relatively to the two thread grooves land 3. During this rolling,the balls 5 will come in no contact with one another if the separators21 are interposed among the balls 5. In addition, the relative slidingspeed between the balls 5 and the separators 21 will be ½ as high as therelative sliding speed between balls in a ball screw device adopting afull ball system (that is, without any separator). Thus, the friction ofthe balls 5 can be suppressed in cooperation of lubricity deriving fromthe material of the separators 21. As a result, expected performance canbe maintained in the ball screw device even if it is operated over along term.

In the ball screw device according to the present invention, separators21 each having a sectional shape shown in FIGS. 7 to 11, FIGS. 12A and12B, FIGS. 13A to 13D and FIGS. 14A to 14D. Incidentally, examples ofseparator materials include resin having a lubricating effect in itself,such as polyamide (e.g. polyamide 66) or fluororesin, or polyethyleneimpregnated with lubricating oil. In addition, a material disclosed inJapanese Patent Application No. 2001-389586 may be used.

The separator 21 shown in FIG. 7 comes in contact with the balls 5 onits opposite surfaces. Each of the surfaces is formed into a concavesurface 23 having a curvature radius R larger than the radius r of eachball 5. The separator 21 configured thus comes in contact with the balls5 with a comparatively small area, and thickness t of the separator 21in the central portion is smaller than thickness L of the separator 21in the outer circumference. Thus, the interval between the balls 5 isreduced. In addition, the thickness t of the separator 21 in the centralportion is small so that the total number of balls 5 is not so muchreduced in comparison with that in a ball screw device adopting the fullball system. Thus, the reduction in load capacity and rigidity can bekept to a minimum. Further, since the concave surface 23 is formed withthe curvature radius R larger than the radius r of each ball 5, thesliding resistance of the ball 5 against the concave surface 23 isreduced. While the sliding resistance is reduced, lubricant such asgrease becomes easy to gain entrance through a gap formed between theouter circumferential edge of the concave surface 23 and the ball 5.Thus, the driving resistance of the screw shaft 2 is also reduced. Inaddition, an elastic preload is generally applied to the balls 5 whenthe ball screw device is assembled. On that occasion, the separators 21are elastically deformed more easily than the balls 5 so that there isanother advantage that the assembling work becomes easy.

The separator 21 shown in FIG. 8 is formed in such a manner that athrough hole 25 communicating with the two concave surfaces 23 of theseparator 21 shown in FIG. 7 is provided additionally in the separator21 shown in FIG. 7, at a location of the central portion of the concavesurfaces 23. According to this separator 21, lubricant 27 is retained inthe through hole 25 so that the lubrication between the separator 21 andthe balls 5 is performed more smoothly. Thus, the abrasion of the balls5 or the driving resistance of the screw shaft 2 can be further reduced.This is because the balls 5 rotate on their axes A-A and A′-A′ and incontact with the male thread groove 1 and the female thread groove 3 onthe front and back paper surfaces parallel to the paper section B-B sothat the lubricant 27 retained in the through hole 25 also gainsentrance between the two thread grooves 1 and 3 by the effect of therotations of the balls 5 on their axes. Thus, more effective lubricationcan be obtained.

In the separator 21 shown in FIG. 9 (sectional view) and FIG. 10(perspective view), a plurality of lips 29 (e.g. three lips 29 at anequal interval as illustrated) extending outward are providedadditionally at the outer circumferential edge of each concave surface23 of the separator 21 shown in FIG. 8. In this separator 21, the lips29 are elastically deformed by the pressure applied from the balls 5 asillustrated by the broken lines (reference numeral 29 a) in FIG. 9,respectively. Thus, it becomes easier to allow the lubricant to gainentrance between the separators 21 and the balls 5 while it becomeseasier to apply an elastic preload to the balls 5 at the time ofassembling. Further, there also occurs a centering action in theseparators 21 with respect to the balls. In addition, since the lips 29are formed at an equal interval (with the rotational phase difference of60° in the illustrated embodiment), it is comparatively easy to producea molding die.

Alternatively, the lips 29 may be formed as separate members in theseparator 21 shown in FIGS. 9 and 10. FIG. 11 shows a sectional view ofthe separator 21 configured thus. For example, the body of the separator21 having the concave surfaces 23 is made of comparatively hard resin,and the lips 29 are made of comparatively soft resin or rubber. Theseseparator body and lips are integrated by an insert injection moldingmethod. Thus, the separator 21 can be formed. In the separator 21 havingthe lips 29 as separate members, the elastic modulus of the lips 29 canbe set desirably. It is therefore easier to apply the elastic preload tothe balls 5, while the centering action occurs more effectively.

Further, as shown in FIGS. 12A and 12B, the separator 21 may be formedas either a separator (FIG. 12A) which has an offset Gothic arch shape,and in which a ratio f_(p)=r_(p)/D_(w) of a curvature radius r_(p) ofthe concave surface 23 to a ball diameter D_(w) is in a range of from0.53 to 0.67, a ratio Γ=A_(p)/D_(w) of an offset A_(p) of the Gothicarch shape to the ball diameter D_(w) is in a range offrom3.486×10⁻¹f_(p)−1.743×10⁻¹ to 3.390f_(p)−1.700, and a ratioθ=d/D_(w) of an outer diameter d of the separator to the ball diameterD_(w) is in a range of from 0.65 to 0.85, or a separator (FIG. 12B) inwhich each of contact portions of the concave surfaces with the ballshas a conical surface shape with an apex angle α not smaller than 64°and not larger than 140°, and a ratio θ=d/D_(w) of an outer diameter dof the separator to the ball diameter D_(w) is in a range of from 0.65to 0.85.

Description will be made specifically. In the separator 21 shown in FIG.12A, the contact portion of each concave surface 23 with the ball 5 hasa Gothic arch shape, having an offset curvature center 30 in its radialdirection and a through hole 25 formed in its central portion. Inaddition, when d (mm) designates the outer diameter of the separator 21,D_(w) (mm) designates the ball diameter, r_(p) (mm) designates thecurvature radius of the concave surface 23, and A_(p) (mm) designatesthe offset of the Gothic arch shape, the concave surface 23 is designedto satisfy the following relationship. That is, the ratiof_(p)=r_(p)/D_(w) of the curvature radius r_(p) to the ball diameterD_(w) is in a range of from 0.53 to 0.67, the ratio Γ=A_(p)/D_(w) of theoffset A_(p) to the ball diameter D_(w) is in a range of from3.486×10⁻¹f_(p)−1.743×10⁻¹ to 3.390f_(p)−1.700, and the ratio θ=d/D_(w)of the outer diameter d to the ball diameter D_(w) is in a range of from0.65 to 0.85. Incidentally, the conditions that the values Γ and f_(p)should satisfy are shown in FIG. 15.

On the other hand, in the separator 21 shown in FIG. 12B, the contactportion of each concave surface 23 with the ball 5 has a conical shape,and further a through hole 25 is formed in the central portion. Inaddition, when d (mm) designates the outer diameter of the separator 21,D_(w) (mm) designates the ball diameter, and α (°) designates the apexangle of the conical surface, the concave surface 23 is designed tosatisfy the following relationship. That is, the apex angle α is notsmaller than 64° and not larger than 140°, and the ratio θ=d/D_(w) is ina range of from 0.65 to 0.85.

When each of the separators 21 shown in FIGS. 12A and 12B has theconcave surface 23 defined as described above, the contact portionbetween the ball 5 and the separator 21 is formed into a ring-like shapeso that the effect of holding the separator 21 in the center between theballs 5 is enhanced. Thus, the balls 5 can be kept at an equal intervaleffectively so that an offset load caused by the displacement of the nut4 relative to the screw shaft 2 is hardly applied to the balls 5.Accordingly, the durability of the linear motion device is improved. Inaddition, since the central portion of the separator 21 is sunk, effectsimilar to that in each separator 21 described previously can beobtained. Accordingly, the ball screw device mounted with the separators21 shown in FIG. 12A or 12B is suitable for use in high contact pressureconditions.

Incidentally, of the concave surface 23, only the contact portion withthe ball 5 may be formed into the Gothic arch or conical surface shape.The concave surface 23 may be formed into any shape if it allows eachball 5 and each separator 21 to come in ring-like contact with eachother so that the balls 5 can be kept at an equal interval. Modificationor improvement may be made desirably within a scope not to hinder theeffect of keeping the balls 5 at an equal interval. For example, asshown in FIGS. 13A to 13D and FIGS. 14A to 14D, the shape of the concavesurface 23 other than its contact portion with the ball 5 may be notformed into a Gothic arch or conical surface shape but formed into acomplex concave surface shape. Incidentally, in FIGS. 13A to 13D andFIGS. 14A to 14D, the reference sign b_(c) designates the ball center,and the reference sign I designates the contact direction and positionbetween the ball 5 and the separator 21. In addition, in FIGS. 13A to13D, the reference sign A designates the conical surface inside thecontact portion; B, the Gothic arch shape portion of the contact portiondefined in the above description; and C, the conical surface outside thecontact portion. In FIGS. 14A to 14D, the reference sign D designatesthe spherical surface inside the contact portion; E, the conical surfaceof the contact portion defined in the above description; and F, thespherical surface outside the contact portion. Each portion other thanthe portions designated by the signs B and E may be formed into anyshape. In addition, each separator 21 may have no through hole 25 in itscentral portion.

The ball screw device according to the present invention is mounted withthe separators 21 having a specific shape described above. In addition,it is preferable that the ratio f=r/D_(w)of the radius r of the rollingelement groove to the ball diameter D_(w) is set to be 0.505-0.550.Particularly, the ratio f is preferably set to be 0.510-0.530. When theratio f exceeds 0.550, the maximum contact pressure between each ball 5and the rolling element groove becomes excessively high so that thedurability of the ball screw device deteriorates. On the contrary, whenthe ratio f is lower than 0.505, the contact ellipse is apt to run ontothe shoulder portion of the rolling element groove. Thus, there is afear that the durability of the ball screw device deteriorates. When theratio f is in the range of 0.505-0.550, the contact ellipse hardly runsonto the shoulder portion of the rolling element groove, and the maximumcontact pressure between each ball 5 and the rolling element groove canbe suppressed to be low. Thus, the durability of the ball screw deviceis improved.

Grease is charged into the ball screw device configured thus. The greasewill be described below in detail.

Base oil of the grease is not limited in kind if the kinematic viscosityof the base oil is in a range of 50-500 mm²/s. Mineral-based lubricatingoil and synthetic lubricating oil can be used preferably. Though notlimited, paraffin-based mineral oil, naphthene-based mineral oil, andmixed oil of these oils may be used as examples of the mineral-basedlubricating oil. Though not limited, synthetic hydrocarbon oil, etheroil, ester oil and fluorine oil may be used as examples of the syntheticlubricating oil. A specific example of synthetic hydrocarbon oil is polya-olefin oil. Specific examples of ether oil may include dialkyldiphenyl ether oil, alkyl triphenyl ether oil, and alkyl tetraphenylether oil. Specific examples of ester oil may include diester oil,neopentyl type polyol ester oil, complex ester oil of those ester oils,and aromatic ester oil. Specific examples of fluorine oil may includeperfluoro ether oil, fluorosilicone oil, chlorotrifluoroethylene oil,and fluorophosphazene oil. Each of these lubricating oils may be usedsingly or in desired combination with another or other kinds of thelubricating oils. Of the lubricating oils, it is preferable that thesynthetic lubricating oil is contained when the lubricating performanceand the life of the lubricating oil at a high temperature and at a highspeed is taken into consideration. Particularly, it is preferable thatester oil or ether oil is used singly as base oil. In addition, from thepoint of view of the cost, it is preferable that the mineral-basedlubricating oil is contained.

When the kinematic viscosity of the base oil at 40° C. is lower than 50mm²/s, the oil film thereof becomes so thin that direct contact of metalwith metal is apt to occur. Thus, the durability deteriorates. On thecontrary, when the kinematic viscosity is higher than 500 mm²/s, theworking torque increases so that the advantage of small friction loss isspoiled. Further, the calorific power increases so that there is a fearthat the durability deteriorates. Particularly, it is preferable thatthe kinematic viscosity of the base oil at 40° C. is in a range of100-500 mm²/s.

Any thickener for the grease can be used without any particularlimitation if the thickener is a substance disperable colloidally in thebase oil to thereby make the base oil semisolid or solid. Examples ofsuch thickener may include metal-soap-based thickener such as lithiumsoap, calcium soap, sodium soap, aluminum soap, lithium complex soap,calcium complex soap, sodium complex soap, barium complex soap oraluminum complex soap; inorganic-based thickener such as bentonite orclay; and organic-based thickener such as monourea compound, diureacompound, triurea compound, tetraurea compound, urethane compound,sodium terephthalate, or calcium sulfonate complex. Of the thickeners,the urea compounds are preferred. In addition, each of these thickenersmay be used singly or in desired combination of another or other kindsof the thickeners.

It is preferable that the content of the thickener is 3-40% by mass,particularly 5-25% by mass, with respect to the total mass of thegrease. Incidentally, when a plurality of kinds of thickeners are used,the total mass of the thickeners is regarded as the aforementionedcontent. When the content of the thickener is lower than 3% by mass, itbecomes difficult to keep the state of the grease. Thus, there is a fearthat the grease flows out. On the contrary, when the content of thethickener is higher than 40% by mass, the grease is too hard to expect asatisfactory lubricating effect, and the torque is increased inevitably.

In addition, various additives may be added to the grease by request inorder to improve various properties. For example, each of an extremepressure agent, an antioxidant, an antirust, an oiliness improver, and ametal deactivator may be added singly or in combination with another orother kinds of the additives within a scope not to spoil the effect ofthe present invention. Known additives may be used. Preferred examplesof the additives will be listed below.

Examples of the antioxidant may include an amine-based antioxidant suchas phenyl-1-naphthylamine; a phenyl-based antioxidant such as2,6-di-t-butylphenol; a sulfur-based antioxidant; and azinc-dithiophosphate-based antioxidant.

Examples of the antirust may include organic sulfonate of alkali metalor alkaline-earth metal; alkyl or alkenyl succinic acid derivative suchas alkyl or alkenyl succinic ester; and partial ester of polyhydricalcohol such as sorbitan monoolate.

Examples of the oiliness improver may include fatty acid andanimal/vegetable oil.

An example of the metal deactivator is benzotriazole.

The extreme pressure agent is particularly a preferred additive when usein high load conditions is taken into consideration. The extremepressure agent shows the highest effect in combination with theseparators 21 shown in FIGS. 12A and 12B, FIGS. 13A to 13D or FIGS. 14Ato 14D. Of the extreme pressure agents, metal salts of organic acidusing metal species such as nickel, molybdenum and tellurium;coordination compound likewise; addition compound likewise; alkyl metalcompound likewise; metal ester compound likewise; and metal alkoxidecompound likewise are preferably used. In addition, as the metal saltsof organic acid, organic carboxylic acid based metal salt, organicsulfur-based metal salt, and organic phosphorus-based metal salt arepreferable. Particularly, metal dithiocarbamate expressed by thefollowing general formula (I) and metal dithiophosphate expressed by thefollowing general formula (II) are preferable.n=2, 3, 4   (I)x, y, z=0, 1, 2, 3, 4   (II)

In the general formulae (I) and (II), M designates nickel, molybdenum ortellurium. In addition, R¹ and R²may be the same or different, eachdesignating alkyl group, cycloalkyl group, alkenyl group, aryl group,alkyl aryl group, or aryl alkyl group. Examples of particularlypreferable group may include 1,1,3,3-tetramethylbutyl group,1,1,3,3-tetramethylhexyl group, 1,1,3-trimethylhexyl group,1,3-dimethylbutyl group, 1-methylundecane group, 1-methylhexyl group,1-methylpentyl, 2-ethylbutyl group, 2-ethylhexyl group,2-methylcyclohexyl group, 3-heptyl group, 4-methylcyclohexyl group,n-butyl group, isobutyl group, isopropyl group, isoheptyl group,isopentyl group, undecyl group, eicosyl group, ethyl group, octadecylgroup, octyl group, cyclooctyl group, cyclododecyl group, cyclopentylgroup, dimethylcyclohexyl group, decyl group, tetradecyl group, docosylgroup, dodecyl group, tridecyl group, trimethylcyclohexyl group, nonylgroup, propyl group, hexadecyl group, hexyl group, henicosyl group,heptadecyl group, heptyl group, pentadecyl group, pentyl group, methylgroup, tertiary butylcyclohexyl group, tertiary butyl group, 2-hexenylgroup, 2-methallyl group, allyl group, undecenyl group, oleyl group,decenyl group, vinyl group, butenyl group, hexenyl group, heptadecenylgroup, tolyl group, ethylphenyl group, isopropylphenyl group, tertiarybutylphenyl group, dibasic pentylphenyl group, n-hexylphenyl group,tertiary octylphenyl group, isononylphenyl group, n-dodecylphenyl group,phenyl group, benzyl group, 1-phenylmethyl group, 2-phenylethyl group,3-phenylethyl group, 1,1-dimethylbenzyl group, 2-phenylisopropyl group,2-phenylhexyl group, benzhydryl group, and biphenyl group. In addition,each of these groups may have ether condensation.

As for metal salt of organic acid other than compounds expressed by thegeneral formulae (I) and (II), metal salt between2-mercaptobenzothiazole and nickel, molybdenum or tellurium ispreferable. Further, metal salt between nickel, molybdenum or telluriumand naphthenic acid or fatty acid is also preferable.

Further, compounds expressed by the following general formula (III) canbe used as the organic nickel compound, the organic molybdenum compoundand the organic tellurium compound.n=2, 3, 4   (III)

In the general formula (III), M designates nickel, molybdenum ortellurium. In addition, R³and R⁴may be the same or different, eachdesignating alkyl group or aryl group.

Each of the organic nickel compound, the organic molybdenum compound andthe organic tellurium compound is added singly or in desired combinationwith the other. The loading of the additive on this occasion is in therange of 0.1-15% by mass with respect to the total mass of the grease.Particularly, the range of 0.5-5% by mass is preferable. When theloading is lower than 0.1% by mass, the additive is too dilute to obtainthe effect of improving the durability. On the contrary, when theloading is higher than 15% by mass, not only is it impossible to obtainthe effect of improving the durability correspondingly to the increaseof the loading, but there is also a fear that the durabilitydeteriorates due to progress of abrasion caused by a chemical action.

Other additives that can be used may include organic zinc compounds suchas zinc dialkyldithiophosphate, zinc diaryldithionate and zincdialkyldithiocarbamate; phosphates; and phosphites.

Any additive other than the aforementioned additives may be added aslong as it does not spoil the performance required of the greaseaccording to the present invention.

The grease configured thus is in a range of 220-395, preferably in arange of 265-350 in worked penetration defined by JIS K2220. When theworked penetration is lower than 220, the grease is too hard to expect asatisfactory lubricating effect. On the contrary, when the workedpenetration is higher than 395, the grease is excessively soft so thatthere is a fear that the grease flows out.

The embodiments of the present invention have described above on thecase where a ball screw device is used by way of example. Similarly, thegrease and the spacers can be applied to linear guide devices or linearball bearing devices which are other linear motion devices. Thus, therespective linear motion devices can be improved in durability.

For example, a linear guide device having the configuration shown inFIG. 5 may be adopted. Preferably the separators 21 shown in FIGS. 7 to11, FIGS. 12A and 12B, FIGS. 13A to 13D and FIGS. 14A to 14D aremounted, and further the grease is charged in the linear guide device.

In addition, a linear ball bearing device having the configuration shownin FIG. 16 may be adopted. The illustrated linear ball bearing devicehas an outer cylinder 41 and a retainer 42. The retainer 42 is receivedinside the outer cylinder 41. A substantially track-like guide groove isformed in the outer circumferential surface of the retainer 42 so as toextend along the axis of the retainer 42. Balls and separators 21preferably shown in FIGS. 7 to 11, FIGS. 12A and 12B, FIGS. 13A to 13Dand FIGS. 14A to 14D are received rollably in the guide groove, andfurther the aforementioned grease is charged therein. In addition, anotch window 43 is opened in the retainer 42 so as to extend along theaxis of the retainer 42, and a part of the balls received in the guidegroove protrude from the notch window 43. Then, the linear ball bearingdevice is fitted onto a linear shaft 45 so as to move the linear shaft45 linearly.

The present invention will be described further with its examples andcomparative examples.

EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLES 1 TO 3

Greases A to F were prepared by compositions shown in Table 1.Incidentally, nickel dithiocarbamate, molybdenum dithiocarbamate andtellurium dithiocarbamate expressed by the general formula (I) were usedas extreme pressure agents in the greases A to F. In addition, in eachof the greases, amine-based antioxidant (PAN) was used as antioxidant,and barium sulfonate was used as antirust. In addition, the kinematicviscosity of used base oil at 40° C. and the worked penetration (JISK2220) of each grease obtained were put down together in Table 1.

Then, by use of ball screw devices (bearing number: BS8020-7.5, shaftdiameter: 80 (mm)) made by NSK Ltd., test ball screw devices weremanufactured with different kinds of separators and different kinds ofgreases as shown in Table 2. The test ball screw devices were operatedcontinuously without feeding oil thereto on the way, and the number ofreciprocating cycles before the occurrence of peeling and the number ofreciprocating cycles before seizure were measured. TABLE 1 Grease RecipeGrease A Grease B Grease C Grease D Grease E Grease F thickener Ureaurea lithium urea urea lithium thickener 8.50 9.80 7.40 6.00 13.60 4.10loading base oil mineral oil mineral oil mineral oil PAO mineral oilmineral oil base oil 460 150 460 410 32 460 kinematic viscositycoefficient extreme 1.00 1.00 1.00 1.00 1.00 1.00 pressure agent loadingantioxidant 1.00 1.00 1.00 1.00 1.00 1.00 loading antirust 1.00 1.001.00 1.00 1.00 1.00 loading worked 310 265 333 350 290 415 penetrationnote 1)loading: % by massnote 2)base oil kinematic viscosity coefficient: mm²/s, 40° C.note 3)worked penetration: JIS K2220

TABLE 2 Comparison of Durability between Examples and ComparativeExamples charged cycle number before cycle number before used separatorgrease occurrence of peeling occurrence of seizure Example 1 concavesphere A 810,000 880,000 (FIG. 7) Example 2 concave sphere with B950,000 1,000,000 through hole (FIG. 8) Example 3 lip integral-type C1,080,000 1,120,000 (FIGS. 9 and 10) Example 4 lip separate-type D1,160,000 1,190,000 (FIG. 11) Comparative none A 520,000 610,000 Example1 Comparative concave sphere E 720,000 770,000 Example 2 (FIG. 7)Comparative lip integral-type F — 570,000 Example 3 (FIGS. 9 and 10)note 1)cycle number: timesnote 2)the cycle number before occurrence of peeling in Comparative Example 3was not measured due to outflow of the grease

The result is shown in Table 2. It is understood that the durability isimproved greatly by charging greases A to D according to the presentinvention.

According to the present invention, occurrence of indentation or peelingin the ball surface and the raceway surface is suppressed and thedurability of a linear motion device is improved.

1. A linear motion device comprising: a linear motion body externallyfitted onto a shaft and making a relative linear motion along the shaft;a plurality of balls retained in a rolling element groove formed in aninner surface of the linear motion body, and rolling between the rollingelement groove and the shaft; separators interposed among the balls; anda circulating path formed in the linear motion body and for circulatingthe balls from one end side of the rolling element groove to the otherend side thereof; wherein the linear motion device is charged withgrease containing base oil which is 220-395 in worked penetrationdefined by JIS K2220 and 50-500 mm²/s in kinematic viscosity at 40° C.2. The linear motion device according to claim 1, wherein surfaces ofeach of the separators in contact with the balls are concave surfaceseach formed with a curvature radius larger than a radius of each of theballs.
 3. The linear motion device according to claim 1, whereinsurfaces of each of the separators in contact with the balls are concavesurfaces each formed with a curvature radius larger than a radius ofeach of the balls, and a through hole formed to penetrate each of theseparators from one of the concave surfaces to the other of the concavesurfaces is formed in a central portion of the separator.
 4. The linearmotion device according to claim 1, wherein surfaces of each of theseparators in contact with the balls are concave surfaces each formedwith a curvature radius larger than a radius of each of the balls, and athrough hole formed to penetrate each of the separators from one of theconcave surfaces to the other of the concave surfaces is formed in acentral portion of the separator, while lips extending outward areformed integrally with the separator at a plurality of locations in acircumferential edge of each of the concave surfaces.
 5. The linearmotion device according to claim 1, wherein surfaces of each of theseparators in contact with the balls are concave surfaces each formedwith a curvature radius larger than a radius of each of the balls, and athrough hole formed to penetrate each of the separators from one of theconcave surfaces to the other of the concave surfaces is formed in acentral portion of the separator, while lips made of separate membersare provided additionally to the separator at a plurality of locationsin a circumferential edge of each of the concave surfaces so as toextend outward. 6-13. (canceled)